/*
 * Copyright (c) 1998, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
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 *
 *
 *
 *
 */

package java.net;

import java.io.*;

/**
 * Utility class for HTML form decoding. This class contains static methods
 * for decoding a String from the <CODE>application/x-www-form-urlencoded</CODE>
 * MIME format.
 * <p>
 * The conversion process is the reverse of that used by the URLEncoder class. It is assumed
 * that all characters in the encoded string are one of the following:
 * &quot;{@code a}&quot; through &quot;{@code z}&quot;,
 * &quot;{@code A}&quot; through &quot;{@code Z}&quot;,
 * &quot;{@code 0}&quot; through &quot;{@code 9}&quot;, and
 * &quot;{@code -}&quot;, &quot;{@code _}&quot;,
 * &quot;{@code .}&quot;, and &quot;{@code *}&quot;. The
 * character &quot;{@code %}&quot; is allowed but is interpreted
 * as the start of a special escaped sequence.
 * <p>
 * The following rules are applied in the conversion:
 *
 * <ul>
 * <li>The alphanumeric characters &quot;{@code a}&quot; through
 * &quot;{@code z}&quot;, &quot;{@code A}&quot; through
 * &quot;{@code Z}&quot; and &quot;{@code 0}&quot;
 * through &quot;{@code 9}&quot; remain the same.
 * <li>The special characters &quot;{@code .}&quot;,
 * &quot;{@code -}&quot;, &quot;{@code *}&quot;, and
 * &quot;{@code _}&quot; remain the same.
 * <li>The plus sign &quot;{@code +}&quot; is converted into a
 * space character &quot; &nbsp; &quot; .
 * <li>A sequence of the form "<i>{@code %xy}</i>" will be
 * treated as representing a byte where <i>xy</i> is the two-digit
 * hexadecimal representation of the 8 bits. Then, all substrings
 * that contain one or more of these byte sequences consecutively
 * will be replaced by the character(s) whose encoding would result
 * in those consecutive bytes.
 * The encoding scheme used to decode these characters may be specified,
 * or if unspecified, the default encoding of the platform will be used.
 * </ul>
 * <p>
 * There are two possible ways in which this decoder could deal with
 * illegal strings.  It could either leave illegal characters alone or
 * it could throw an {@link java.lang.IllegalArgumentException}.
 * Which approach the decoder takes is left to the
 * implementation.
 *
 * @author Mark Chamness
 * @author Michael McCloskey
 * @since 1.2
 */

public class URLDecoder {

  // The platform default encoding
  static String dfltEncName = URLEncoder.dfltEncName;

  /**
   * Decodes a {@code x-www-form-urlencoded} string.
   * The platform's default encoding is used to determine what characters
   * are represented by any consecutive sequences of the form
   * "<i>{@code %xy}</i>".
   *
   * @param s the {@code String} to decode
   * @return the newly decoded {@code String}
   * @deprecated The resulting string may vary depending on the platform's default encoding.
   * Instead, use the decode(String,String) method to specify the encoding.
   */
  @Deprecated
  public static String decode(String s) {

    String str = null;

    try {
      str = decode(s, dfltEncName);
    } catch (UnsupportedEncodingException e) {
      // The system should always have the platform default
    }

    return str;
  }

  /**
   * Decodes a {@code application/x-www-form-urlencoded} string using a specific
   * encoding scheme.
   * The supplied encoding is used to determine
   * what characters are represented by any consecutive sequences of the
   * form "<i>{@code %xy}</i>".
   * <p>
   * <em><strong>Note:</strong> The <a href=
   * "http://www.w3.org/TR/html40/appendix/notes.html#non-ascii-chars">
   * World Wide Web Consortium Recommendation</a> states that
   * UTF-8 should be used. Not doing so may introduce
   * incompatibilities.</em>
   *
   * @param s the {@code String} to decode
   * @param enc The name of a supported <a href="../lang/package-summary.html#charenc">character
   * encoding</a>.
   * @return the newly decoded {@code String}
   * @throws UnsupportedEncodingException If character encoding needs to be consulted, but named
   * character encoding is not supported
   * @see URLEncoder#encode(java.lang.String, java.lang.String)
   * @since 1.4
   */
  public static String decode(String s, String enc)
      throws UnsupportedEncodingException {

    boolean needToChange = false;
    int numChars = s.length();
    StringBuffer sb = new StringBuffer(numChars > 500 ? numChars / 2 : numChars);
    int i = 0;

    if (enc.length() == 0) {
      throw new UnsupportedEncodingException("URLDecoder: empty string enc parameter");
    }

    char c;
    byte[] bytes = null;
    while (i < numChars) {
      c = s.charAt(i);
      switch (c) {
        case '+':
          sb.append(' ');
          i++;
          needToChange = true;
          break;
        case '%':
                /*
                 * Starting with this instance of %, process all
                 * consecutive substrings of the form %xy. Each
                 * substring %xy will yield a byte. Convert all
                 * consecutive  bytes obtained this way to whatever
                 * character(s) they represent in the provided
                 * encoding.
                 */

          try {

            // (numChars-i)/3 is an upper bound for the number
            // of remaining bytes
            if (bytes == null) {
              bytes = new byte[(numChars - i) / 3];
            }
            int pos = 0;

            while (((i + 2) < numChars) &&
                (c == '%')) {
              int v = Integer.parseInt(s.substring(i + 1, i + 3), 16);
              if (v < 0) {
                throw new IllegalArgumentException(
                    "URLDecoder: Illegal hex characters in escape (%) pattern - negative value");
              }
              bytes[pos++] = (byte) v;
              i += 3;
              if (i < numChars) {
                c = s.charAt(i);
              }
            }

            // A trailing, incomplete byte encoding such as
            // "%x" will cause an exception to be thrown

            if ((i < numChars) && (c == '%')) {
              throw new IllegalArgumentException(
                  "URLDecoder: Incomplete trailing escape (%) pattern");
            }

            sb.append(new String(bytes, 0, pos, enc));
          } catch (NumberFormatException e) {
            throw new IllegalArgumentException(
                "URLDecoder: Illegal hex characters in escape (%) pattern - "
                    + e.getMessage());
          }
          needToChange = true;
          break;
        default:
          sb.append(c);
          i++;
          break;
      }
    }

    return (needToChange ? sb.toString() : s);
  }
}
